Control systems for controlling the excess-air factor lambda (.lambda.) have been known for a long time and are described in pertinent literature in detail. Particularly, German published patent application DE-OS No. 3,036,107 (U.S. Pat. No. 4,440,737) discloses an adaptive lambda control arrangement for a fuel metering system for an internal combustion engine wherein, in addition to the already existing control system, multiplicative and additive correction quantities are formed and stored in nonvolatile memory stores. In the lower part-load range and at idling, the control arrangement permits an additive, and in the upper part-load range and under full-load conditions, a multiplicative regulation of the lambda shift.
By these means, the anticipatory control of the lambda value is gradually adapted to the varying operating parameters of the internal combustion engine. This special type of adaptation as disclosed in DE-OS No. 3,036,107 is based on the realization that in the anticipatory control of the lambda value substantially additive errors occur at a low load of the internal combustion engine, whereas the errors are substantially multiplicative at a high load of the internal combustion engine.
Additive errors may be caused especially by so-called leakage air portions which are portions of air that are not detected by the load sensor, for example, an air flow sensor. Multiplicative errors may result from temperature or pressure variations, for example, relating to the density of the fuel or the intake air quantity. Thus, such an adaptation of the anticipatory control obviates the need for an altitude sensor since altitude-dependent density errors are compensated for automatically.
On the whole, this arrangement has proven to be satisfactory although for some operating ranges of the internal combustion engine optimum conditions are not yet present. As investigations have shown, a further drift possibility which is not covered by the two above-described correcting possibilities must not be neglected. The reason for this is that the known control arrangement only considers additive speed-independent errors. Although in the event of an additive speed-dependent error, the control system is in a position to correct the error for a specific predetermined speed, the correction value just determined will no longer be correct when moving into a new speed range so that the correction procedure starts anew. In general, however, the engine speed varies so rapidly that the adaptive adjustment with its relatively large control time constant falls out of step. Emission tests have shown that such an error may mislead the adaptive control, causing the exhaust gas to become less clean than would be the case without adaptive control.